U.S. patent application number 10/921503 was filed with the patent office on 2006-02-23 for thoracic impedance detection with blood resistivity compensation.
This patent application is currently assigned to Cardiac Pacemakers, Inc.. Invention is credited to John Hatlestad, Boyce Moon, Jeffrey E. Stahmann.
Application Number | 20060041280 10/921503 |
Document ID | / |
Family ID | 35910611 |
Filed Date | 2006-02-23 |
United States Patent
Application |
20060041280 |
Kind Code |
A1 |
Stahmann; Jeffrey E. ; et
al. |
February 23, 2006 |
Thoracic impedance detection with blood resistivity
compensation
Abstract
This document discusses, among other things, a cardiac rhythm
management device or other implantable medical device that uses
thoracic impedance to determine how much fluid is present in the
thorax, such as for detecting or predicting congestive heart
failure, pulmonary edema, pleural effusion, hypotension, or the
like. The thoracic fluid amount determined from the thoracic
impedance is compensated for changes in blood resistivity, which
may result from changes in hematocrit level or other factors. The
blood-resistivity-compensated thoracic fluid amount can be stored
in the device or transmitted to an external device for storage or
display. The blood-resistivity-compensated thoracic fluid amount
can also be used to adjust a cardiac pacing, cardiac
resynchronization, or other cardiac rhythm management or other
therapy to the patient. This document also discusses applications
of the devices and methods for predicting or indicating anemia.
Inventors: |
Stahmann; Jeffrey E.;
(Ramsey, MN) ; Hatlestad; John; (Maplewood,
MN) ; Moon; Boyce; (Ham Lake, MN) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH
1600 TCF TOWER
121 SOUTH EIGHT STREET
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Cardiac Pacemakers, Inc.
|
Family ID: |
35910611 |
Appl. No.: |
10/921503 |
Filed: |
August 19, 2004 |
Current U.S.
Class: |
607/17 ; 600/484;
600/547 |
Current CPC
Class: |
A61B 5/053 20130101;
A61N 1/36521 20130101; A61N 1/3627 20130101 |
Class at
Publication: |
607/017 ;
600/484; 600/547 |
International
Class: |
A61N 1/36 20060101
A61N001/36; A61B 5/02 20060101 A61B005/02; A61B 5/05 20060101
A61B005/05 |
Claims
1. A method comprising: detecting a thoracic impedance signal of a
thorax of a subject; detecting a blood resistivity signal; and
determining a thoracic fluid indication using the thoracic
impedance signal, the determining the thoracic fluid indication
including using the blood resistivity signal to reduce or eliminate
an effect of a change in the blood resistivity on the thoracic
fluid indication.
2. The method of claim 1, further comprising filtering the thoracic
impedance signal to obtain a near-DC thoracic impedance signal, and
in which the thoracic fluid indication is generated at least in
part by using the near-DC thoracic impedance signal.
3. The method of claim 2, in which the filtering the thoracic
impedance signal includes attenuating or removing frequency
components above about 0.05 Hz.
4. The method of claim 2, in which the filtering the thoracic
impedance signal includes attenuating or removing a cardiac stroke
component of the thoracic impedance signal.
5. The method of claim 2, in which the filtering the thoracic
impedance signal includes attenuating or removing a respiration
component of the thoracic impedance signal.
6. The method of claim 2, in which the filtering the thoracic
impedance signal includes attenuating or removing a posture
component of the thoracic impedance signal.
7. The method of claim 2, in which the filtering the thoracic
impedance signal includes using a variable lowpass cutoff frequency
filter.
8. The method of claim 1, in which the detecting the thoracic
impedance signal and the determining the thoracic fluid indication
are performed using an implantable medical device.
9. The method of claim 1, in which the detecting the thoracic
impedance signal comprises: delivering a test current from first
and second implantable electrodes; and measuring a resulting
voltage in response to the test current from third and fourth
implantable electrodes.
10. The method of claim 9, in which at least one of the first and
second implantable electrodes is the same electrode as the third
and fourth implantable electrodes.
11. The method of claim 9, in which the first and second
implantable electrodes are different electrodes from the third and
fourth implantable electrodes.
12. The method of claim 9, in which the delivering the test current
includes delivering a carrier signal.
13. The method of claim 1, in which the detecting the thoracic
impedance signal includes using at least one cardiovascular
electrode.
14. The method of claim 1, in which the detecting the thoracic
impedance signal includes using at least one electrode located on a
housing of an implantable medical device.
15. The method of claim 1, in which the detecting the thoracic
impedance signal includes using at least one electrode located on a
substantially electrically insulating header of an implantable
medical device.
16. The method of claim 1, in which the detecting the blood
resistivity signal includes using first and second intravascular
electrodes.
17. The method of claim 1, in which the detecting the blood
resistivity signal includes: delivering a test current between
first and second electrodes, in which the first and second
electrodes are both located within single heart chamber or blood
vessel; and measuring a resulting voltage in response to the test
current.
18. The method of claim 1, in which the detecting the blood
resistivity includes measuring a volume of blood that is
substantially independent of a degree of fluid present in the
thorax.
19. The method of claim 1, in which the detecting the blood
resistivity includes measuring the blood resistivity at like times
during at least one of a cardiac cycle and a respiratory cycle.
20. The method of claim 1, in which the determining a thoracic
fluid indication using the thoracic impedance signal includes:
measuring a first thoracic impedance value at a first time;
measuring a first blood resistivity value at a second time that is
close to the first time; measuring a second thoracic impedance
value at a third time; and measuring a second blood resistivity
value at a fourth time that is close to the third time; and
normalizing, using the first and second blood resistivity values,
the second thoracic impedance value to the first thoracic impedance
value.
21. The method of claim 20, in which the normalizing includes
multiplying the second thoracic impedance value by a ratio of the
first and second blood resistivity values.
22. The method of claim 1, in which the determining a thoracic
fluid indication using the thoracic impedance signal includes:
measuring a baseline first blood resistivity value; measuring a
first thoracic impedance value and a corresponding second blood
resistivity value; and computing a second thoracic impedance value
using the first thoracic impedance value and the first and second
blood resistivity values.
23. The method of claim 22, in which the computing the second
thoracic impedance value includes multiplying the first thoracic
impedance value by a ratio of the first blood resistivity value to
the second blood resistivity value.
24. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and determining whether heart failure
decompensation is present using the history of thoracic fluid
measurements to determine whether a change in thoracic fluid has
occurred.
25. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and predicting heart failure
decompensation using the history of the thoracic fluid measurements
to determine whether a change in thoracic fluid has occurred.
26. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and determining whether pulmonary
edema is present using the history of thoracic fluid measurements
to determine whether a change in thoracic fluid has occurred.
27. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and predicting pulmonary edema using
the history of thoracic fluid measurements to determine whether a
change in thoracic fluid has occurred.
28. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and determining whether pleural
effusion is present using the history of thoracic fluid
measurements to determine whether a change in thoracic fluid has
occurred.
29. The method of claim 1, further comprising: storing a history of
thoracic fluid measurements; and predicting pleural effusion using
the history of thoracic fluid measurements to determine whether a
change in thoracic fluid has occurred.
30. The method of claim 1, further including transmitting
information about the thoracic fluid indication from an implantable
medical device to an external device.
31. The method of claim 30, further including displaying the
information about the thoracic fluid indication on the external
device.
32. The method of claim 30, further including storing the
information about the thoracic fluid indication in the external
device.
33. The method of claim 1, further including providing therapy to
the subject, the therapy determined at least in part using the
thoracic fluid indication.
34. The method of claim 1, in which the using the blood resistivity
signal to reduce or eliminate an effect of a change in the blood
resistivity on the thoracic fluid indication includes: measuring a
posture of the patient; storing a first baseline blood resistivity
corresponding to a first posture; storing a second baseline blood
resistivity corresponding to a second posture that is different
from the first posture; and compensating the thoracic fluid
indication by selecting and using the one of the first and second
baseline blood resistivities that corresponds to the then-current
posture of the patient.
35. The method of claim 34, in which the first posture is an
upright posture and the second posture is a recumbent posture.
36. A method of using an implantable medical device, the method
comprising: detecting a thoracic impedance signal of a thorax of a
subject using implantable electrodes to deliver a test current and
measure a responsive voltage; detecting a blood resistivity signal,
within a heart chamber or blood vessel, from a volume of blood that
is substantially independent of a cardiac cycle and a respiratory
cycle; filtering the thoracic impedance signal to substantially
remove frequency components above 0.05 Hz to obtain a near-DC
thoracic impedance signal; and determining a thoracic fluid
indication using the thoracic impedance signal, the determining the
thoracic fluid indication including compensating the thoracic
impedance signal for changes in blood resistivity.
37. The method of claim 36, in which the compensating comprises
multiplying a thoracic impedance signal value by a ratio of a
baseline blood resistivity to a blood resistivity measured close in
time to a time when the thoracic impedance signal value was
measured.
38. The method of claim further comprising: storing a history of
thoracic fluid measurements; and determining whether at least one
of heart failure decompensation, pulmonary edema, and pleural
effusion is present using the history of thoracic fluid
measurements to determine whether a change in thoracic fluid has
occurred.
39. A system comprising: an implantable medical device, including:
a thoracic impedance measurement circuit, to provide a thoracic
impedance signal; a blood resistivity measurement circuit, to
provide a blood resistivity signal; and a controller, coupled to
the blood resistivity measurement circuit, the controller operable
to determine a thoracic fluid indication using the filtered
thoracic impedance signal, including using the blood resistivity
signal to reduce or eliminate an effect of a change in the blood
resistivity on the thoracic fluid indication.
40. The system of claim 39, in which the thoracic impedance
measurement circuit includes: a test current circuit to deliver a
test current to first and second implantable electrodes; and a
voltage measurement circuit to measure a resulting voltage between
third and fourth implantable electrodes.
41. The system of claim 40, further including the at least one of
the first and second implantable electrodes and at least one of the
third and fourth implantable electrodes, and in which at least one
of the first and second implantable electrodes is the same
electrode as the third and fourth implantable electrodes.
42. The system of claim 40, further including the at least one of
the first and second implantable electrodes and at least one of the
third and fourth implantable electrodes, and in which the first and
second implantable electrodes are different electrodes from the
third and fourth implantable electrodes.
43. The system of claim 40, in which the test current circuit
includes a carrier signal generator circuit.
44. The system of claim 39, further including at least one
cardiovascular leadwire electrode to detect the thoracic impedance
signal.
45. The system of claim 39, in which the implantable medical device
includes a housing, and in which the housing includes at least one
electrode to detect the thoracic impedance signal.
46. The system of claim 45, in which implantable medical device
includes at least one substantially electrically insulating header
attached to the housing, and in which the header includes at least
one electrode to detect the thoracic impedance signal.
47. The system of claim 39, further comprising first and second
cardiovascular electrodes to detect the blood resistivity
signal.
48. The system of claim 47, in which the first and second
cardiovascular electrodes are arranged with respect to each other
to be disposed within the same heart chamber or blood vessel.
49. The system of claim 47, further including a sense amplifier to
detect intrinsic heart signals corresponding to each cardiac cycle
of a subject, and in which the blood resistivity measurement
circuit includes a synchronization circuit to synchronize blood
resistivity measurements to at least one of (1) like phases of
different cardiac cycles; and (2) like phases of different
respiration cycles.
50. The system of claim 39, further including a frequency selective
filter circuit, to receive the thoracic impedance signal, the
frequency selective filter circuit providing a near-DC filtered
thoracic impedance signal.
51. The system of claim 50, in which the filter circuit includes a
lowpass filter circuit including at least one lowpass pole at a
frequency of about 0.05 Hz.
52. The system of claim 50, in which the filter circuit includes a
lowpass filter circuit including at least one lowpass pole at a
frequency that attenuates or removes a cardiac stroke component of
the thoracic impedance signal.
53. The system of claim 50, in which the filter circuit includes a
lowpass filter circuit including at least one lowpass pole at a
frequency that attenuates or removes a respiration component of the
thoracic impedance signal.
54. The system of claim 50, in which the filter circuit includes a
lowpass filter circuit including at least one variable-frequency
lowpass pole.
55. The system of claim 39, further comprising: a posture sensor,
to provide a posture signal; and a posture compensation module,
operable to compensate the thoracic fluid indication using the
posture signal.
56. The system of claim 55, in which the posture compensation
module includes different baseline blood resistivity measurements
corresponding to different postures.
57. The system of claim 39, in which the controller includes: a
first memory location to store a first thoracic impedance value
measured at a first time; a second memory location to store a first
blood resistivity value measured at a second time that is close to
the first time; a third memory location to store a second thoracic
impedance value measured at a third time; and a fourth memory
location to store a second blood resistivity value measured at a
fourth time that is close to the second time; and a normalization
module to normalize, using the first and second blood resistivity
values, the second thoracic impedance value to the first thoracic
impedance value.
58. The system of claim 57, in which the normalization module
includes an arithmetic logic unit (ALU) that multiplies the second
thoracic impedance value by a ratio of the first and second blood
resistivity values.
59. The system of claim 39, in which the controller circuit
includes: an averaging circuit to measure a first blood resistivity
value; a first memory location to store a first thoracic impedance
value; a second memory location to store a second blood resistivity
value that corresponds to the first thoracic impedance value; and
in which the controller circuit is operable to compute a second
thoracic impedance value using the first thoracic impedance value
and the first and second blood resistivity values.
60. The system of claim 59, in which the controller includes a
multiplier circuit to multiply the first thoracic impedance value
by a ratio of the first blood resistivity value to the second blood
resistivity value.
61. The system of claim 39, further comprising: a memory configured
to store a history of thoracic fluid measurements; and a heart
failure decompensation detection or prediction module operable to
determine whether heart failure decompensation is present or likely
to occur using the history of thoracic fluid measurements to
determine whether a change in thoracic fluid has occurred.
62. The system of claim 61, in which the memory and the
decompensation detection or prediction module are located in the
implantable medical device.
63. The system of claim 61, in which the memory and the
decompensation detection or prediction module are located in an
external device.
64. The system of claim 39, further comprising: a memory configured
to store a history of thoracic fluid measurements; and a pulmonary
edema detection or prediction module operable to determine whether
pulmonary edema is present or likely to occur using the history of
thoracic fluid measurements to determine whether a change in
thoracic fluid has occurred.
65. The system of claim 64, in which the memory and the pulmonary
edema detection or prediction module are located in the implantable
medical device.
66. The system of claim 64, in which the memory and the pulmonary
edema detection or prediction module are located in an external
device.
67. The system of claim 39, further comprising: a memory configured
to store a history of thoracic fluid measurements; and a pleural
effusion detection or prediction module operable to determine
whether pleural effusion is present or likely to occur using the
history of thoracic fluid measurements to determine whether a
change in thoracic fluid has occurred.
68. The system of claim 67, in which the memory and the pleural
effusion detection or prediction module are located in the
implantable medical device.
69. The system of claim 67, in which the memory and the pleural
effusion detection or prediction module are located in an external
device.
70. The system of claim 39, further including an external device
operable to be communicatively coupled to the implantable medical
device to receive information about the thoracic fluid indication
from the implantable medical device.
71. The system of claim 70, in which the external device includes a
display device to display the information about the thoracic fluid
indication.
72. The system of claim 70, in which the external device includes a
storage device to store the information about the thoracic fluid
indication.
73. The system of claim 39, further including a therapy circuit,
coupled to the controller, the therapy circuit operable to provide
a therapy to a subject, the therapy determined at least in part
using the thoracic fluid indication from the controller.
74. A system comprising: an implantable medical device, including:
a thoracic impedance measurement circuit, to provide a thoracic
impedance signal; a frequency-selective filter circuit, coupled to
the thoracic impedance measurement circuit to receive the thoracic
impedance signal, the frequency selective filter circuit including
at least one lowpass pole providing a near-DC filtered thoracic
impedance signal, the at least one lowpass pole substantially
attenuating or removing at least one of a heart contraction
component and a respiration component of the thoracic impedance
signal; a blood resistivity measurement circuit, to provide a blood
resistivity signal from which a blood resistivity measurement is
obtained, the blood resistivity measurement being substantially
independent of a variation resulting from heart contractions,
respiration, and degree of thoracic fluid; and a controller,
coupled to the filter circuit and the blood resistivity measurement
circuit, the controller operable to determine a thoracic fluid
indication using the filtered thoracic impedance signal, including
using the blood resistivity signal to reduce or eliminate an effect
of a change in the blood resistivity on the thoracic fluid
indication, the thoracic fluid indication determined by multiplying
a thoracic impedance signal value by a ratio of a baseline blood
resistivity to a blood resistivity measured close in time to a time
when the thoracic impedance signal value was measured.
75. The system of claim 74, further comprising: a memory storage
device operable to store a history of thoracic fluid measurements;
and means for determining whether at least one of heart failure
decompensation, pulmonary edema, and pleural effusion is present
using the history of thoracic fluid measurements to determine
whether a change in thoracic fluid has occurred.
76. A system comprising: an implantable medical device, including:
a thoracic impedance measurement circuit, to provide a thoracic
impedance signal; a frequency selective filter circuit, coupled to
the thoracic impedance measurement circuit to receive the thoracic
impedance signal, the frequency selective filter circuit providing
a near-DC filtered thoracic impedance signal; a blood resistivity
measurement circuit, to provide a blood resistivity signal; means
for determining a thoracic fluid indication using the filtered
thoracic impedance signal; and means for reducing or eliminating an
effect of a change in blood resistivity on the thoracic fluid
indication.
77. The system of claim 76, further comprising: a memory storage
device operable to store a history of thoracic fluid measurements;
and means for determining whether at least one of heart failure
decompensation, pulmonary edema, and pleural effusion is present
using the history of thoracic fluid measurements to determine
whether a change in thoracic fluid has occurred.
78. A method comprising: detecting a first blood resistivity at a
first time using at least one implanted intravascular or
intracardiac electrode in a patient; detecting a second blood
resistivity at a second time using the at least one implanted
intravascular or intracardiac electrode; comparing the second blood
resistivity to the first blood resistivity; and if the second blood
resistivity is less than the first blood resistivity by at least a
threshold value, then declaring an indication or prediction of
anemia to be present in the patient.
79. The method of claim 78, in which the detecting the first blood
resistivity and the detecting the second blood resistivity are
performed in the same blood vessel or heart chamber.
80. The method of claim 78, in which the detecting the first blood
resistivity and the detecting the second blood resistivity are
performed under like conditions with respect to at least one of a
cardiac cycle, a respiratory cycle, a posture, and a circadian
cycle.
81. The method of claim 78, further comprising communicating an
indication of anemia predicted or anemia present to an external
device.
82. A system comprising: an implantable medical device, including:
first and second electrodes located within the same blood vessel or
heart chamber; a blood resistivity measurement circuit, to provide
a blood resistivity signal obtained between the first and second
electrodes at different first and second times; and a controller,
coupled to the blood resistivity measurement circuit, the
controller operable to determine an indication or prediction of
anemia by comparing a current blood resistivity to a stored
baseline blood resistivity value.
83. The system of claim 82, in which the blood resistivity
measurement circuit includes: a test current circuit to deliver a
test current to first and second implantable electrodes; and a
voltage measurement circuit to measure a resulting voltage between
third and fourth implantable electrodes.
84. The system of claim 82, further comprising a heart signal
sensing circuit configured to synchronize the first and second
times to like portions of different cardiac cycles.
85. The system of claim 82, further comprising a thoracic impedance
measurement circuit configured to synchronize the first and second
times to like portions of different respiration cycles.
86. The system of claim 82, further comprising a posture sensor and
a posture compensation module configured to store different
baseline blood resistivities corresponding to different postures
obtained from the posture sensor.
Description
TECHNICAL FIELD
[0001] This document pertains generally to implantable medical
devices and more particularly, but not by way of limitation, to
congestive heart failure (CHF) thoracic fluid detection and other
thoracic impedance systems, devices, or methods that compensate or
correct for changes in blood resistivity.
BACKGROUND
[0002] Variations in how much fluid is present in a person's thorax
can take various forms and can have different causes. Eating salty
foods can result in retaining excessive fluid in the thorax and
elsewhere. Posture changes can also affect the amount of thoracic
fluid. For example, moving from supine to standing can shift
intravascular fluid away from the thorax toward the lower
extremities.
[0003] Another example is pulmonary edema, which results in buildup
of extravascular fluid in the lungs. In pulmonary edema, fluid
accumulates in extracellular spaces, such as the spaces between
lung tissue cells. One cause of pulmonary edema is congestive heart
failure (CHF), which is also sometimes referred to as "chronic
heart failure," or as "heart failure." CHF can be conceptualized as
an enlarged weakened portion of heart muscle. The impaired heart
muscle results in poor cardiac output of blood. As a result of such
poor blood circulation, blood tends to pool in blood vessels in the
lungs. This intravascular fluid buildup, in turn, results in the
extravascular fluid buildup mentioned above. In sum, pulmonary
edema can be one important condition associated with CHF.
[0004] Yet another example of thoracic fluid accumulation is
pleural effusion, which is the buildup of extravascular fluid in
the space between the lungs and the rib cage. Pleural effusion can
also result from CHF because, as discussed above, intravascular
fluid buildup can result in the extravascular interstitial fluid
buildup. The extravascular fluid buildup of pulmonary edema can, in
turn, result in the extravascular fluid buildup of pleural
effusion.
[0005] CHF may also activate several physiological compensatory
mechanisms. Such compensatory mechanisms are aimed at correcting
the reduced cardiac output. For example, the heart muscle may
stretch to increase its contractile power. Heart muscle mass may
also increase. This is referred to as "hypertrophy." The ventricle
may also change its shape as another compensatory response. In
another example, a neuro-endocrine response may provide an
adrenergic increase in heart rate and contraction force. The
Renin-Angiotensin-Aldosterone-System (RAAS) may be activated to
induce vasoconstriction, fluid retention, and redistribution of
blood flow. Although the neuro-endocrine response is compensatory,
it may overload the cardiovascular system. This may result in
myocardial damage, and may exacerbate CHF.
[0006] Diagnosing CHF may involve physical examination,
electrocardiogram (ECG), blood tests, chest radiography, or
echocardiography. Managing a CHF patient is challenging. CHF may
require potent drugs. Moreover, treatment may be thwarted by the
compensatory mechanisms, which may recompensate for the presence of
the medical treatment. Therefore, treating CHF involves a delicate
balance to properly manage the patient's hemodynamic status in a
state of proper compensation to avoid further degeneration.
[0007] However, this delicate balance between compensation and
effective CHF treatment is easily upset, even by seemingly benign
factors, such as common medication (e.g., aspirin), physiological
factors, excitement, or gradual progression of the disease. This
may plunge the patient into a decompensation crisis, which requires
immediate corrective action so as to prevent the deterioration of
the patient's condition which, if left unchecked, can lead to
death. In sum, accurately monitoring the symptoms of CHF, such as
thoracic fluid accumulation, is very useful for avoiding such a
decompensation crisis and properly managing the CHF patient in a
state of relative well-being.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] In the drawings, which are not necessarily drawn to scale,
like numerals describe substantially similar components throughout
the several views. Like numerals having different letter suffixes
represent different instances of substantially similar components.
The drawings illustrate generally, by way of example, but not by
way of limitation, various embodiments discussed in the present
document.
[0009] FIG. 1 is a block diagram illustrating generally one example
of a system that provides a thoracic fluid amount indication that
is adjusted to compensate for a change in blood resistivity, if
any.
[0010] FIG. 2 is a schematic illustration of one example in which
portions of the system are implemented in an implantable cardiac
rhythm management (CRM) or other implantable medical device
(IMD).
[0011] FIG. 3 is a block diagram illustrating generally another
example in which portions of the system are implemented in an
implantable CRM or other IMD.
[0012] FIG. 4 is a flow chart illustrating generally one example of
a method of providing a thoracic fluid amount indication that is
compensated for any changes in blood resistivity.
[0013] FIG. 5 is a flow chart illustrating generally one example of
a method of detecting anemia using a blood impedance measurement
performed by an implantable medical device.
DETAILED DESCRIPTION
[0014] The following detailed description includes references to
the accompanying drawings, which form a part of the detailed
description. The drawings show, by way of illustration, specific
embodiments in which the invention may be practiced. These
embodiments, which are also referred to herein as "examples," are
described in enough detail to enable those skilled in the art to
practice the invention. The embodiments may be combined, other
embodiments may be utilized, or structural, logical and electrical
changes may be made without departing from the scope of the present
invention. The following detailed description is, therefore, not to
be taken in a limiting sense, and the scope of the present
invention is defined by the appended claims and their
equivalents.
[0015] In this document, the terms "a" or "an" are used, as is
common in patent documents, to include one or more than one. In
this document, the term "or" is used to refer to a nonexclusive or,
unless otherwise indicated. Furthermore, all publications, patents,
and patent documents referred to in this document are incorporated
by reference herein in their entirety, as though individually
incorporated by reference. In the event of inconsistent usages
between this document and those documents so incorporated by
reference, the usage in the incorporated reference(s) should be
considered supplementary to that of this document; for
irreconcilable inconsistencies, the usage in this document
controls.
[0016] In this document, the term intravascular includes the term
intracardiac.
[0017] In this document, the term cardiovascular includes an
association with either the heart or a blood vessel.
[0018] In this document, the term "thorax" refers to a human
subject's body other than the subject's head, arms, and legs.
[0019] FIG. 1 is a block diagram illustrating generally one example
of a system 100 that provides an indication of the amount of fluid
in the thorax ("thoracic fluid indication") that is adjusted to
compensate for a change in blood resistivity, if any.
[0020] In this example, the system 100 includes a thoracic
impedance measurement circuit 102. The thoracic impedance
measurement circuit 102 receives at least one electrical signal
from electrodes associated with a patient's thorax. This electrical
signal is typically received in response to a test energy applied
to the thorax, such as by a thoracic impedance test energy delivery
circuit 104.
[0021] One illustrative example of some electrode configurations
and circuits for performing thoracic impedance measurements is
described in Hartley et al. U.S. Pat. No. 6,076,015 entitled RATE
ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE USING TRANSTHORACIC
IMPEDANCE, which is assigned to Cardiac Pacemakers, Inc., and which
is incorporated herein by reference in its entirety, including its
description of performing thoracic impedance measurements. The
Hartley et al. U.S. Pat. No. 6,076,015 uses thoracic impedance to
obtain a respiration signal. By contrast, the present patent
application uses thoracic impedance to obtain a thoracic fluid
status signal. Therefore, the signal of interest in the present
patent application would be deemed noise in the Hartley et al. U.S.
Pat. No. 6,076,015, and vice-versa. However, both thoracic fluid
status and respiration are obtainable using the thoracic impedance
detection techniques described in the Hartley et al. U.S. Pat. No.
6,076,015. The present thoracic fluid status signal of interest is
obtained from a lower frequency (i.e., a "near-DC") portion of the
thoracic impedance signal rather than the frequencies of the
respiration signal described in the Hartley et al. U.S. Pat. No.
6,076,015. In this document, the "near-DC" component of the
thoracic impedance signal refers to the frequencies below which
respiration and cardiac contractions significantly influence the
thoracic impedance signal. This near-DC component of the thoracic
impedance signal, therefore, typically refers to signal frequencies
below a cutoff frequency having a value of about 0.1 Hz, such as at
signal frequencies between about 5.times.10.sup.-7 Hz and 0.05 Hz,
because the cardiac stroke and respiration components of the
thoracic impedance signal lie at higher frequencies. Fluid
accumulation in the thorax corresponds to a decrease in the near-DC
thoracic impedance. Conversely, fluid depletion in the thorax
corresponds to an increase in the near-DC thoracic impedance. As
discussed above, fluid accumulation may result from, among other
things, pulmonary edema or pleural effusion, both of which may
result from CHF.
[0022] In the example of FIG. 1, the system 100 also includes a
controller 108. The controller 108 is typically a microprocessor or
any other circuit that is capable of sequencing through various
control states such as, for example, by using a digital
microprocessor having executable instructions stored in an
associated instruction memory circuit, a microsequencer, or a state
machine. In this example, the controller 108 includes a digital
signal processor (DSP) circuit 110. The digital signal processor
circuit 110 performs any digital filtering or other signal
processing needed to extract from the thoracic impedance signal a
near-DC desired thoracic fluid amount signal. The digital signal
processor circuit 110, therefore, may implement one or more filter
circuits, and such filter circuits may be implemented as a sequence
of executable instructions, rather than by dedicated filtering
hardware.
[0023] However, the present inventors have recognized that the
near-DC thoracic impedance signal is typically also affected by
confounding factors other than the amount of fluid present in the
thorax. One such confounding factor is any change in blood
resistivity. Blood resistivity changes as a function of hematocrit
in the blood. The hematocrit (Ht) or packed cell volume (PCV) is
the proportion of blood that is occupied by red blood cells. It is
typically between 0.35 and 0.52, and is slightly higher on average
in males than in females. For example, when a patient is
dehydrated, there will be less fluid in the patient's blood.
Therefore, the patient's hematocrit level will increase, that is,
the patient's blood will include a higher percentage of other
components, such as insulative red blood cells. This will increase
the blood resistivity, which, in turn, will affect the thoracic
impedance signal even through it is not necessarily associated with
the extravascular fluid accumulation of pulmonary edema or pleural
effusion. Other factors that are believed to possibly influence
blood resistivity include the patient's electrolyte level, certain
medications in the blood, proteins in the blood, or blood gas
concentrations.
[0024] As an illustrative example, the above change in hematocrit
percentage from 35% to 52% may correspond to a change in
resistivity from about 140 .OMEGA.cm to about 200 .OMEGA.-cm. Such
changes in blood resistivity will influence the near-DC thoracic
impedance measurement. This will confound an extravascular thoracic
fluid amount determination using the near-DC thoracic impedance
measurement, unless the extravascular thoracic fluid amount
determination is corrected for such variations in blood
resistivity, if any. Measurement of variations in blood resistivity
is typically affected by the frequency of the excitation signal
that are used. At higher excitation frequencies, blood cells
typically become more resistive.
[0025] Accordingly, the system in FIG. 1 illustrates a blood
resistivity measurement circuit 106. The blood impedance
measurement circuit 106 receives a blood impedance measurement from
electrodes that are associated with blood (and preferably blood in
the thorax) such as in response to a delivery of test energy by a
blood impedance test energy delivery circuit 112. In one example,
the blood impedance measurement circuit 106 and the blood impedance
test energy delivery circuit 112 are configured similar to the
thoracic impedance measurement circuit 102 and the thoracic
impedance test energy delivery circuit 104, respectively, as
discussed above, except for being connected to different
electrodes. Using the blood impedance measurement, the controller
108 executes a sequence of instructions to compute a blood
resistivity correction 114. The blood resistivity correction 114 is
applied to the thoracic fluid indication that is output by the
digital signal processor circuit 110. This yields an adjusted
thoracic fluid amount indication 116.
[0026] In FIG. 1, the thoracic impedance test energy delivery
circuit 104 is illustrated separately from the blood impedance test
energy delivery circuit 112 to assist the reader's
conceptualization. In practice, these circuits, or portions
thereof, may be combined. The combined circuit may be coupled to
different electrodes for delivering the thoracic impedance test
energy than for delivering the blood impedance test energy.
Similarly, in FIG. 1, the thoracic impedance measurement circuit
102 is illustrated separately from the blood impedance test energy
delivery circuit 112 to assist the reader's conceptualization. In
practice, these circuits, or portions thereof, may be combined. The
combined circuit may be coupled to different electrodes for
measuring the responsive voltages for the thoracic and blood
impedance measurements, as discussed below.
[0027] FIG. 2 is a schematic illustration of one example in which
portions of the system 100 are implemented in an implantable
cardiac rhythm management (CRM) or other implantable medical device
(IMD) 200. In this example, the IMD 200 is coupled to a heart 202
using at least one leadwire, such as a multielectrode leadwire 204.
In this example, the leadwire 204 includes a tip electrode 206, a
distal ring electrode 208, and a proximal ring electrode 210, each
of which is disposed in the right ventricle of the heart 202. In
this example, each of the tip electrode 206, the distal ring
electrode 208, and the proximal ring electrode 210 is independently
electrically connected to a corresponding separate electrically
conductive terminal within an insulating header 212. The header 212
is affixed to a housing 214 carrying electronic components of the
IMD 200. In this example, the header 212 includes a header
electrode 216, and the housing 214 includes a housing electrode
218.
[0028] In one example, thoracic impedance is sensed by delivering a
test current between: (1) at least one of the ring electrodes 208
or 210; and (2) the housing electrode 218, and a resulting
responsive voltage is measured across the tip electrode 206 and the
header electrode 216. Because the IMD 200 is typically pectorally
implanted at some distance away from the heart 202, this electrode
configuration injects the test current over a substantial portion
(but typically not the entire portion) of the patient's thorax,
such that when the resulting voltage measurement is divided by the
test current magnitude, it yields an indication of thoracic
impedance. Using different electrodes for delivering the current
and for measuring the responsive voltage reduces the component of
the measured impedance signal that results from ohmic losses in the
leadwires to the test current delivery electrodes. While such a
"four-point" probe is useful, it is not required. In other
examples, a "three-point probe" (having three electrodes, with one
electrode used for both test current delivery and responsive
voltage measurement), or a "two-point probe" (having two
electrodes, each electrode used for both test current delivery and
responsive voltage measurement) are used. Moreover, other electrode
combinations could alternatively be used to implement a four-point
probe. The above four-point probe description provides an
illustrative example of one suitable four-point probe
configuration.
[0029] In one example, blood impedance is sensed by delivering a
test current between: (1) one of the distal ring electrode 208 or
the proximal ring electrode 210; and (2) the housing electrode 218.
A resulting responsive voltage is measured between: (1) the other
of the distal ring electrode 208 or the proximal ring electrode
210; and (2) the tip electrode 206. In this example, although the
test current is injected across a substantial portion of the
patient's thorax, as discussed above, the responsive voltage signal
of interest is measured across electrodes within the same chamber
of the patient's heart (or, alternatively, within the same blood
vessel). Therefore, when the responsive voltage measurement is
divided by the test current magnitude, it yields an indication of
the blood impedance in the heart chamber rather than the thoracic
impedance. The measured blood impedance is used to compensate the
measured thoracic impedance for changes in the blood impedance.
[0030] FIG. 3 is a block diagram illustrating generally another
example in which portions of the system 100 are implemented in an
implantable CRM or other IMD 300. The example of FIG. 3 includes an
impedance test stimulus circuit 302 that, together with an
impedance measurement circuit 304, provides thoracic and blood
impedance measurements. In response to one or more control signals
from the controller 108, an electrode configuration multiplexer 306
couples these circuits to the appropriate electrodes for the
particular thoracic or blood impedance measurement. In this
example, the multiplexer 306 is also coupled to a heart signal
sensing circuit 308, which includes sense amplifier or other
circuits for detecting from particular electrodes intrinsic
electrical heart signals that include electrical depolarizations
corresponding to heart contractions. The multiplexer 306 is also
coupled to a therapy circuit 310, such as a pulse delivery circuit
for delivering pacing, cardioversion, or defibrillation energy to
particular electrodes in response to one or more control signals
received from the controller 108.
[0031] In the example of FIG. 3, the DSP circuit 110 processes the
thoracic impedance measurements from the impedance measurement
circuit 304. The DSP circuit 110 extracts a cardiac stroke signal
or a respiration signal from the thoracic impedance signal, such as
by using techniques described in the above-incorporated Hartley et
al. U.S. Pat. No. 6,076,015. One or both of the extracted cardiac
stroke or respiration signals is provided to a blood impedance
measurement synchronization circuit 312. The synchronization
circuit 312 includes one or more peak-detector, level-detector, or
zero-cross detector circuits to synchronize the blood impedance
measurement to the same sample point of a cardiac contraction cycle
or a respiration cycle. This reduces the effect of variations in
one or both of these cycles on the blood impedance measurement.
Similarly, the measurements can be taken under the same conditions
with respect to posture or circadian cycle to reduce those effects
on the blood impedance measurement. Posture can be detected using
an accelerometer or other posture sensor; circadian cycle can be
ascertained from a time-of-day indication provided by a clock
circuit within the controller 108. Alternatively, the cardiac cycle
information is extracted from the heart signal sensing circuit 308,
either by itself or in combination with information from the
controller 108 about when pacing or other stimulus pulses that
evoke a responsive heart contraction are issued. The controller 108
computes an adjusted thoracic fluid indication 116 from the
measured thoracic impedance. The adjusted thoracic fluid indication
116 is compensated for blood resistivity variations using the blood
resistivity correction 114 obtained using the measured blood
impedance. In a further example, the implantable medical device 300
includes a telemetry circuit 314 that communicates one of the
blood-resistivity-compensated thoracic impedance or the blood
resistivity and thoracic impedance measurements to an external
programmer 316 or the like for further processing, storage, or
display.
[0032] FIG. 4 is a flow chart illustrating generally one example of
a method of providing a thoracic fluid amount indication that is
compensated for any changes in blood resistivity. At 400, a
thoracic impedance is detected. This may be accomplished in a
number of different ways. In one illustrative example, such as
described in Hartley et al. U.S. Pat. No. 6,075,015, it includes
injecting a four-phase carrier signal, such as between a housing
electrode 218 and a ring electrode 208. In this example, the first
and third phases are +320 microampere pulses that are 20
microseconds long. The second and fourth phases are -320
microampere pulses that are 20 microseconds long. The four phases
are repeated at 50 millisecond intervals to provide a carrier test
current signal from which a responsive voltage can be measured.
However, as discussed elsewhere in this document, because blood
resistivity varies with excitation frequency, a different
excitation frequency may also be used.
[0033] The Hartley et al. U.S. Pat. No. 6,075,015 describes an
exciter circuit for delivering such a test current stimulus
(however, the present system can alternatively use other suitable
circuits, including an arbitrary waveform generator that is capable
of operating at different frequencies or of mixing different
frequencies to generate an arbitrary waveform). It also describes a
signal processing circuit for measuring a responsive voltage
between a housing electrode 216 and a tip electrode 206. In one
example, the signal processing circuit includes a preamplifier,
demodulator, and bandpass filter for extracting the thoracic
impedance data from the carrier signal, before conversion into
digital form by an A/D converter. Further processing is performed
digitally, and is performed differently in the present system 100
than in the Hartley et al. U.S. Pat. No. 6,075,015.
[0034] For example, the Hartley et al. U.S. Pat. No. 6,075,015
includes a bandpass filter that receives the output of the A/D
converter. The purpose of the highpass portion of the bandpass
filter is to attenuate the near-DC portion of the thoracic
impedance signal, which is the signal of interest to the present
system 100. Therefore, the present system 100 eliminates the
highpass filter. The cutoff frequency of the remaining lowpass
filter is selected to pass the near-DC portion of the thoracic
impedance signal and attenuate higher frequency portions of the
thoracic impedance signal, including the respiration and cardiac
stroke components of the thoracic impedance signal. In one example,
a programmable cutoff frequency lowpass filter is used. In another
example, an adaptive cutoff frequency lowpass filter is used, such
that the cutoff frequency is moved to a higher frequency for higher
values of heart rate and respiration frequency, and the cutoff
frequency is moved to a lower frequency for lower values of heart
rate and respiration frequency.
[0035] At 402 of FIG. 4, blood impedance is detected and measured.
There are a number of ways in which this can be done. In one
example, the blood impedance measurement is performed in the same
manner as the thoracic impedance measurement, except that
measurement of the responsive voltage is across two electrodes that
are both typically located in the same heart chamber or same blood
vessel, such as between (1) one of the distal ring electrodes 208
or the proximal ring electrode 210; and (2) the other of the distal
ring electrode 208 or the proximal ring electrode 210. Because the
blood impedance is to be used to correct a thoracic fluid
indication, it is typically detected and measured at or near the
thorax. Alternatively, however, even an external blood impedance
measurement could be used, if desired. In one example, the blood
impedance is sampled under appropriate other conditions (e.g., at a
like point in different cardiac cycles, at a like point in
different respiration cycles, etc.).
[0036] At 404, a thoracic fluid amount indication is determined.
There are a number of ways in which this can be done. In one
example, the thoracic fluid amount indication is given by the value
of the near-DC thoracic impedance signal, which may be averaged or
otherwise filtered, if desired. In another example, a baseline
value of this averaged or otherwise filtered near-DC thoracic
impedance signal is obtained from the patient, and the thoracic
fluid amount indication is given by the difference of the near-DC
thoracic fluid impedance value (with the same or different
averaging or filtering) from this baseline value.
[0037] At 406, the thoracic fluid amount indication obtained from
the near-DC thoracic impedance is adjusted to compensate for
changes in blood resistivity. In one example, the adjusted thoracic
fluid amount indication is given by:
TFA.sub.adj=TFA.sub.raw(.rho..sub.Blood, current)/(.rho..sub.Blood,
baseline). In this equation, TFA.sub.adj is the adjusted value of
the thoracic fluid amount, (.rho..sub.Blood, baseline) is the
baseline value of the blood resistivity, and (.rho..sub.Blood,
current) is the current value of the blood resistivity. In the
present case, since the same electrodes are used for both the
baseline and current blood resistance measurements, the resistivity
ratio (.rho..sub.Blood, current)/(.rho..sub.Blood, baseline) is
given by the corresponding ratio of the blood resistances, i.e.,
(Z.sub.Blood, current)/(Z.sub.Blood, baseline).
[0038] In a further example, such as where the implantable medical
device 300 optionally includes a posture sensor or detector 318, a
separate baseline impedance or resistivity is provided for
different postures, since posture affects thoracic impedance
measurements. In one example, a separate baseline impedance or
resistivity is stored for upright postures (e.g., sitting or
standing) than for recumbent postures (e.g., supine, prone, left
lateral decubitus, right lateral decubitus). In a further example,
a separate baseline impedance or resistivity is stored for one or
more of the different subtypes of upright or recumbent postures. In
compensating the thoracic fluid amount indication, the posture
compensation module 320 compensates a particular resistivity
measurement by using a baseline resistivity that corresponds to the
then-current posture indicated by the postures detector 318. One
example of a suitable posture detector 318 is a commercially
available two-axis accelerometer, such as Model No. ADXL202E,
manufactured by Analog Devices, Inc. of Norwood, Mass., USA.
[0039] The compensated thoracic fluid amount indication can be
stored in the implantable medical device 300 or transmitted to the
external device 316. Moreover, in one example, the implantable
medical device 300 or external device 316 is capable of storing a
history of the values of the thoracic fluid amount indication to
assist the physician in managing the CHF state of the patient. In
one example, the external device 316 is capable of displaying a
graph, histogram or other chart of such thoracic fluid amount
values.
[0040] In a further example, the implantable medical device 300 or
the external device 316 determines whether heart failure
decompensation, pulmonary edema, or pleural effusion is present,
such as by comparing an increase in the
blood-resistivity-compensated thoracic fluid amount indication to a
corresponding first threshold value to deem one or more of these
conditions to be present.
[0041] In yet a further example, the implantable medical device 300
or the external device 316 predicts whether heart failure
decompensation, pulmonary edema, or pleural effusion is likely to
become present in the future, such as by comparing an increase in
the blood-resistivity-compensated thoracic fluid amount indication
to a corresponding second threshold value to deem one or more of
these conditions to be likely in the future. The second threshold
used for the condition prediction may be different from the first
threshold used for the condition detection. In one example, the
second threshold value reflects a smaller increase in the thoracic
fluid amount indication than the first threshold value.
[0042] In yet a further example, the implantable medical device 300
adjusts a therapy to the patient using the thoracic fluid amount
indication. In one example, an increase in the thoracic fluid
amount indication triggers an increase in a rate at which pacing
pulses are delivered to the heart. In another example, a change in
the thoracic fluid amount indication results in altering another
programmable parameter of cardiac pacing or cardiac
resynchronization therapy, such as, for example, atrioventricular
(AV) delay, particular cardiac stimulation sites, interventricular
delay, or intraventricular delay. In a further example, a change in
the thoracic fluid amount indication triggers the providing of a
warning or other indication to the patient to adjust a medication
level (for example, a diuretic).
[0043] Another application for the present systems, devices, and
methods is in anemia detection. Anemia is a pathological condition
that is often present in CHF patients. Diagnosing and treating
anemia will improve a patient's cardiac function. Therefore, there
is a need to detect anemia in CHF patients, for example, to
communicate a diagnosis regarding the anemia status to a CHF
patient's health care provider.
[0044] FIG. 5 is a flow chart illustrating generally one example of
an anemia detection method. At 500, a baseline blood impedance is
established. In one example, this includes obtaining one or more
near-DC blood impedance measurements (typically taken within the
same blood vessel or heart chamber) such as described above with
respect to 402 of FIG. 4. In one example, the baseline blood
impedance is established by computing a central tendency (e.g.,
average, median, low-pass filtered, etc.) value of a series of such
measured blood impedances over a desired time interval (for
example, one month).
[0045] At 502, a current blood impedance is measured. This near-DC
blood impedance measurement is typically performed in the same
manner and location as described above for establishing the
baseline. At 504, the current blood impedance is compared to the
baseline blood impedance. As described above, a higher percentage
of red blood cells tends to increase blood impedance. Therefore,
when the current blood impedance falls far enough below the
baseline blood impedance, then anemia may be indicated. Therefore,
in one example, if the current blood impedance falls below the
baseline blood impedance by at least an offset threshold value,
then anemia is declared to be present at 506. In one example, the
offset value is a fixed or programmable percentage of the baseline
blood impedance (e.g., 5%, 10%, 20%, etc.). The offset value is
typically set to prevent normal physiological variations in blood
impedance from triggering an anemia detection. In another example,
such as by choosing a different threshold value, the comparison
predicts that anemia is likely to occur (e.g., if the blood
impedance falls at least 10% below its baseline value, then future
anemia is predicted; if the blood impedance then falls at least 20%
below its baseline value, then present anemia is declared).
[0046] In one further example, if anemia is predicted or declared
present, that information is telemetered or otherwise communicated
to the patient's health care provider from the implantable medical
device, such as by providing such information to an external device
for storage or display. In one example, such communication takes
place the next time that the implantable medical device is
interrogated by a programmer or other external interface. In
another example, such the implantable medical device itself
initiates a telemetric or other communication of such information
to an external device. In yet a further example, an anemia warning
is provided to the patient, either directly by the implantable
medical device (e.g., an audible warning), or via an external
interface device.
[0047] As discussed above, measurement of variations in blood
resistivity is typically affected by the frequency of the
excitation signal that are used. At higher excitation frequencies,
blood cells typically become more resistive. Therefore, to yield a
more sensitive measurement of anemia, it may be desirable to use a
higher excitation frequency than would be used for detecting
thoracic impedance, and for correcting the resulting thoracic
impedance measurements for changes in blood resistivity.
Alternatively, such as for measuring thoracic fluid status, if a
change in blood resistivity exceeds a certain threshold value then,
in one example, the system automatically switches to a lower
excitation frequency that is affected less by changes in blood
resistivity.
[0048] Although much of the above discussion has emphasized
correcting near-DC thoracic impedance measurements to account for
changes in blood resistivity, hematocrit-related blood resistivity
changes may also affect higher frequency components of the thoracic
impedance signal (e.g., respiration components, cardiac stroke
components, etc.), somewhat analogous to the way in which patient
posture can affect such higher frequency components of the thoracic
impedance signal. Aspects of the present blood resistivity
measurement and correction techniques may also be used for
correcting such higher-frequency components of the thoracic
impedance signal. However, the effects of blood resistivity changes
at higher frequency may be nonlinear, making correction with a
single multiplicative correction factor difficult or impossible.
Therefore, a nonlinear correction function may be used, if needed.
Such a nonlinear correction function may be empirically determined.
In one example, the nonlinear correction function may be
implemented as a lookup table. Moreover, the higher-frequency
components of the thoracic impedance signal may be used to infer
thoracic fluid status even much of the above discussion focused on
particular examples that extract a thoracic fluid status signal
from the near-DC component of the thoracic impedance signal.
[0049] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. Many other embodiments will be
apparent to those of skill in the art upon reviewing the above
description. The scope of the invention should, therefore, be
determined with reference to the appended claims, along with the
full scope of equivalents to which such claims are entitled. In the
appended claims, the terms "including" and "in which" are used as
the plain-English equivalents of the respective terms "comprising"
and "wherein." Also, in the following claims, the terms "including"
and "comprising" are open-ended, that is, a system, device,
article, or process that includes elements in addition to those
listed after such a term in a claim are still deemed to fall within
the scope of that claim. Moreover, in this document and in the
following claims, the terms "first," "second," and "third," etc.
are used merely as labels, and are not intended to impose numerical
requirements on their objects.
* * * * *